1. Field of the Invention
The present invention relates generally to semiconductor laser chips, and more particularly to a method and apparatus for testing bare laser chips that do not require a submount, carrier or wire bonding of the laser chips.
2. Description of the Related Art
A large number of applications utilize solid state lasers and amplifiers to generate or amplify light at specific wavelengths. Increasingly, laser diodes are employed for communications and are integral to optical disc recording and storage systems.
During production and fabrication of semiconductor laser chips, reliability tests are typically performed by the manufacturer on the semiconductor laser chips to screen out potential reliability hazards. Such testing typically requires the temperature of the laser chip be maintained at a predetermined temperature during the test. For example, present reliability testing of semiconductor lasers includes a test known as a purge of the laser chips. A laser chip has a threshold current I.sub.th at which it will lase, i.e., emit coherent light. To maintain reliability over the life of the laser chip, it is desirable for the threshold current I.sub.th to remain stable. A laser chip may have an initial threshold current of approximately 10 mA, for example. After the laser chip has been subjected to a purge, the threshold current I.sub.th at which a laser chip will lase may change. A change in the threshold current I.sub.th after the purge has been completed has been statistically shown to have a direct correlation to the long-term age rate, and hence reliability, of a laser chip. If the threshold current changes by more than some predetermined amount after the, purge, the expected life span of a laser chip can be statistically determined, and those laser chips which do not have a determined expected life span within a predetermined range will be discarded as unreliable. Thus, an upper limit for the change in threshold current I.sub.th after a purge is determined at which a laser chip will be considered unreliable.
A purge is typically performed by passing 150 mA through the laser chip, while maintaining the temperature of the laser chip at 85.degree. C., for a period of 12 or 24 hours. After the purge is performed, the threshold current I.sub.th of the laser chip is measured and compared to the threshold current I.sub.th for the laser chip before the purge. If the threshold current I.sub.th has changed by more than the upper limit, the laser chip will be considered unreliable and be discarded. For example, the upper limit for a change in the threshold current I.sub.th may be 3 mA. Thus, if the laser chip with an initial threshold current of 10 mA has for example a threshold current of 13 mA or more after the purge, it will be considered unreliable and discarded.
FIG. 1 illustrates in block diagram form a typical mounting assembly necessary for performing a purge on a laser chip 10. Laser chip 10 is solder bonded to a submount 12, such as for example a silicon substrate. Submount 12 is soldered to a carrier 14, such as a copper carrier, which facilitates good heat sinking of the laser chip 10. Wire bonds 16 must be attached from the laser chip 10 to the submount 12. The laser chip 10 is then subjected to a purge as described above.
There are problems, however, with the conventional mounting of a laser chip 10 to perform tests such as a purge. Every laser chip that is tested must be mounted as described with respect to FIG. 1 above, i.e., soldered to a submount 12 (which is in turn soldered to a carrier 14), and attached by wire bonds 16. If the laser chip 10 passes the purge, the laser chip 10/submount 12 assembly must be de-mounted from the carrier 14 and re-mounted to a final package. If the laser chip 10 does not pass the purge, the laser chip 10/submount 12 assembly must still be de-mounted from the carrier 14 (and discarded) so that the carrier 14 can be reused. In either case the process of de-mounting is labor intensive, thus adding to manufacturing costs. Additionally, if the laser chip 10 fails the purge, the submount 12 is discarded with the laser chip 10, thus increasing manufacturing costs for wasted parts.
The purge assembly process as described above has been streamlined by eliminating the need for the carrier 14 by providing heat sinking from the test device, or alternatively pulsing the purge current to the laser chip, thus allowing the purge to be performed with the laser chip 10 mounted on just the submount 12. While this eliminates one process and handling step (mounting the chip/submount to the carrier), it does not address the issue of the labor required for the wire bonding and the wasted surmounts for laser chips that do not pass the purge.
Thus, there exists a need for an apparatus and method for performing tests on laser chips that are not labor intensive and will not result in wasted parts should the laser chip fail the test.